Convection in Condensible-rich Atmospheres

TL;DR

This paper develops a new convection parameterization applicable to both dilute and nondilute condensible atmospheres, addressing limitations of existing models and exploring implications for planetary climate regimes like runaway greenhouse states.

Contribution

A simple, energy-conserving convection scheme valid for nondilute atmospheres is derived and tested through radiative-convective simulations, extending climate modeling capabilities.

Findings

The scheme accurately captures nondilute convection dynamics.

Latent heat in nondilute atmospheres dampens seasonal temperature variations.

Results demonstrate the scheme's applicability to runaway greenhouse scenarios.

Abstract

Condensible substances are nearly ubiquitous in planetary atmospheres. For the most familiar case-water vapor in Earth's present climate-the condensible gas is dilute, in the sense that its concentration is everywhere small relative to the noncondensible background gases. A wide variety of important planetary climate problems involve nondilute condensible substances. These include planets near or undergoing a water vapor runaway and planets near the outer edge of the conventional habitable zone, for which CO2 is the condensible. Standard representations of convection in climate models rely on several approximations appropriate only to the dilute limit, while nondilute convection differs in fundamental ways from dilute convection. In this paper, a simple parameterization of convection valid in the nondilute as well as dilute limits is derived and used to discuss the basic character of nondilute convection. The energy conservation properties of the scheme are discussed in detail and are verified in radiative-convective simulations. As a further illustration of the behavior of the scheme, results for a runaway greenhouse atmosphere for both steady instellation and seasonally varying instellation corresponding to a highly eccentric orbit are presented. The latter case illustrates that the high thermal inertia associated with latent heat in nondilute atmospheres can damp out the effects of even extreme seasonal forcing.